exposition format: How to deal with duplicate metric families

[beorn7]

The current spec is crystal clear that duplicate metric families (and, correspondingly, multiple TYPE and HELP lines for the same metric name in the text format) are not allowed in a single exposition.

However, the federation endopint of the prometheus server is currently in breach of that, see. prometheus/prometheus#1963 .

This issue is about whether we should allow duplicate metric families, and if yes, how to deal with them.
(Problematic right now is that the Prometheus server tolerates them if ingesting protobuf, and just ingests them one after another, while it rejects any text format exposition with multiple TYPE or HELP lines for the same metric name.)

[beorn7]

To clarify: prometheus/prometheus#1963 is fixed, so federation output is not in breach of the spec anymore.